This is described by Mozart’s father, Leopold Mozart, in a letter to his wife:

[Sent from Rome, dated April 14, 1770. Only parts of the letter relevant to the transcription episode are given here.]

We arrived here safely on the 11th at noon. I could have been more easily persuaded to return to Salzburg than to proceed to Rome, for we had to travel for five days from Florence to Rome in the most horrible rain and cold wind. I am told here that they have had constant rain for four months and indeed we had a taste of it, as we went on Wednesday and Thursday in fine weather to Saint Peter’s and to the Sistine Chapel to hear the Miserere during the mass, and on our way home were surprised by such a frightful downpour that our cloaks have never yet been so drenched as they then were…

You have often heard of the famous Miserere in Rome, which is so greatly prized that the performers in the chapel are forbidden on pain of excommunication to take away a single part of it, to copy it or to give it to anyone. But we have it already. Wolfgang has written it down and we would have sent it to Salzburg in this letter, if it were not necessary for us to be there to perform it. But the manner of performance contributes more to its effect than the composition itself. So we shall bring it home with us. Moreover, as it is one of the secrets of Rome, we do not wish to let it fall into other hands, ut non incurramus mediate vel immediate in censuram Ecclesiae.

—translation by Emily Anderson, in her The Letters of Mozart and his Family; London: Macmillan, 1938.

After playing Chopin, I feel as if I had been weeping over sins that I had never committed, and mourning over tragedies that were not my own. — Oscar Wilde, 1891

Le concert, c’est moi. — Franz Liszt, writing to the Princess Belgiojoso on his launch of a new kind of public concert: the solo recital; quoted by Alfred Brendel in The New York Review of Books (22 Nov 1990)

A smasher of pianos. — Clara Schumann on Liszt, quoted in Alan Walker, Robert Schumann: the Man and his Music (1972)

O Mozart, immortal Mozart, how many, how infinitely many inspiring suggestions of a finer, better life have you left in our souls! — Franz Schubert, Diary, 1816

The sonatas of Mozart are unique; they are too easy for children, and too difficult for artists. — Arthur Schnabel

Mozart in his music was probably the most reasonable of the world’s great composers. It is the happy balance between flight and control, between sensibility and self-discipline, simplicity and sophistication of style that is his particular province… Mozart tapped once again the source from which all music flows, expressing himself with a spontaneity and refinement and breath-taking rightness that has never since been duplicated. — Aaron Copland, Copland on Music (1960)

It is sobering to think that when Mozart was my age he had already been dead a year — Tom Lehrer (speaking of Mozart’s early death at the age of just 35 years)

His character was a mixture of tenderness and coarseness, sensuality and candour, sociability and melancholy. — Johann Mayrhofer on Schubert; quoted in Westrup, Schubert Music (1969)

Mozart is the highest, the culminating point that beauty has attained in the sphere of music.

Tchaikovsky

A world that has produced a Mozart is a world worth saving. What a picture of a better world you have given us, Mozart!

Schubert

Beethoven I take twice a week, Haydn four times, and Mozart every day!

Rossini

Mozart’s music is the mysterious language of a distant spiritual kingdom, whose marvelous accents echo in our inner being and arouse a higher, intensive life. E.T.A. Hoffman

Melody is the essence of music. I compare a good melodist to a fine racer, and counterpointists to hack post-horses; therefore be advised, let well alone and remember the old Italian proverb: ‘Chi sa più, meno sa— Who knows most, knows least. Mozart (spoken to actor Michael Kelly)

I have always reckoned myself among the greatest admirers of Mozart, and shall do so till the day of my death.